The health service in Northern Ireland has no idea how much money is owed by citizens from other countries who have received medical treatment here, the Belfast Telegraph has learned.

Deputy chairman of the Stormont's health committee Jim Wells (right) described the revelation as "extremely disturbing".

The DUP MLA added: "This would come to a serious amount of money."

The black hole in health service finance was exposed when the Belfast Telegraph discovered that trusts here have never had any system in place to record details of people not entitled to free treatment in the province.

This should have been happening so that the relevant authorities in their place of residence can be invoiced for treatment received here.

Insiders says the health service has found it impossible to calculate the amount outstanding, though there is widespread agreement it could be hundreds of millions over the decades.

The news comes after this paper revealed that medical card fraud was costing the Department of Health up to a £100m a year.

At the time Mr Wells said: "People from the Irish Republic are perfectly entitled to come across the border and obtain medical treatment. Then the Department of Health would bill the Irish Republic the full economic cost of that."

The same principle applies to other EU member states.

A spokesman for the Health Department referred to the situation at the Northern Trust.

He said: "In previous years there was no system to capture this information electronically at point of entry to the Northern Health and Social Care Trust.

"The Department of Health has recently issued guidance on charging visitors from overseas who are not entitled to free health care. As a result, we are currently piloting systems to collect this information and support the trust in recouping costs.

"I think you will find other trusts are doing the same."

However, staff at one of the biggest accident and emergency departments in the country confirmed there was no system, and never had been.

When asked about a new system being introduced, all staff on duty stated they had never heard of it.

Mr Wells has undertaken to ask for the introduction of a proper system to collate the information in the Assembly "if necessary by the introduction of a Bill".

He said: "A lot of money is being lost to a department under pressure which has seen a 1.9% increase in funding against a 5.6% increase in demand.

"When I think of all the people entitled to a service here who can't get it and people having to wait for operations only to have them cancelled at the last minute, and here we have yet more millions going out the door that should be used for medical services.

"There's no problem with treating people – but it must be paid for."

How medical card system works on both sides of border

By Victoria O'Hara

People living in the Republic are not entitled to free healthcare north of the border.

If someone from another EU country needs treatment while in the UK – for example on holiday – they will recieve this free of charge, but their country should be billed. The situation is the same for someone from the UK visiting the Republic.

Someone who lives in the South but who works in Northern Ireland is also entitled to an NHS medical card.

They are entitled to GP services, but that does not usually extend to their family.

A UK medical card is entirely different to a medical card in the Republic.

It is not means-tested and is not linked to social welfare payments.

Provided a couple from the South but working in Northern Ireland have no other income sourced in Ireland, they are also entitled to Irish medical cards which would enable them and dependent children to access free public healthcare in the South.

A recent review in the Republic revealed that just 68% of medical cardholders were deemed as eligible to hold the cards in 2012.

After 109,000 reviews were carried out, 74,900 cardholders had their eligibility confirmed.

Here, if you lose your medical card, a replacement can be provided either by visiting the GP or by contacting the health authority.

To register with a doctor or a dentist in Northern Ireland you need a medical card.

This is done by filling in the HS22X form, which is found in surgeries and online.

After filling it in a card will be issued.

This contains an NHS number and other information such as your name, address, date of birth, and details of your registered practice or individual doctor.